2 Answers
2

Yes, you may.
I contacted Microsoft support and they confirmed this.
However, I don't know how much trust I would put into a chat representative for legal concerns. So, I've provided some more information...

The Clip Art and Media gallery
provides a compilation of artwork. See
the use terms for the description of
permitted uses. If those terms do not
meet your needs, our Clip Art partners
at Office Online provide a variety of
images you can license directly.

In the absence of language to the
contrary in the License Agreement,
Sample Art (which includes images
customarily found in the "sample"
folders within Microsoft operating
systems) may be used for personal use
only. You may not sell, lease, or
distribute Sample Art, or any
materials you create that use Sample
images, for any commercial purposes.

Additionally, here is an excerpt straight from PowerPoint 2007 EULA:

Media Elements and Templates. You may
copy and use images, clip art,
animations, sounds, music, shapes,
video clips and templates provided
with the software and identified for
such use in documents and projects
that you create. You may distribute
those documents and projects
noncommercially. If you wish to use
these media elements or templates for
any other purpose, go to
www.microsoft.com/permission to learn
whether that use is allowed.

You may find out additional details here.
For any further inquires I was suggested to contact microsoft directly, in writing, at:

Note that "commercial use" is an annoyingly vague term. Interpreted broadly, it might cover even such seemingly innocuous things as using an image on a website that also has an ad banner on it, or indeed using it for any purpose at a for-profit company.
–
Ilmari KaronenFeb 14 '12 at 21:33